The present invention relates to a high-pressure sodium lamp with high color rendition.
There have been recently devised and demonstrated high-pressure sodium lamps with a high general color rendering index R.sub.a of 65 to 85. Radiation from these lamps are almost equal in color and warmness to incandescent lamps and have by far a higher efficacy and a lamp life than the latter so that they now find wide applications instead of incandescent lamps in various fields.
In general, the high color-rendition, high-pressure sodium lamps of the type described use a straight arc tube whose inner diameter is larger than that of the conventional high-pressure sodium lamp and the operating vapor pressure of sodium filled in the arc tube is raised so that the spectrum covers the whole visible range. However, they have some problems. First of all, radiation at the ends of the arc tube is bluish. Secondly, the starting time is relatively longer.
In order to overcome these and other problems, the inventors disclose in their Japanese Patent Application No. 129780/1980 a high-pressure sodium lamp in which, instead of a straight arc tube, an arc envelope or tube is employed whose inner diameter is largest at the midpoints between the ends and is reduced in the vicinity of both electrodes. This arc envelope is referred to as "an ellipsoidal envelope or tube" in this specification.
The high color-rendition, high-pressure sodium lamps of the types described above which can be used instead of incandescent lamps have a fatal common defect in that their lamp efficacy is considerably low as compared with metal-halide lamps. That is, the conventional lamps with a color temperature of 2500.degree. K. and a wattage of 150, 250 and 400 W exhibit an initial efficacy of about 52, 54 and 58 lm/W (lumens per watt), respectively, which are considerably lower as compared with an efficacy of 70 to 100 lm/W of the metal halide lamps. The same inventors tried to provide high color-rendition, high-pressure sodium lamps of 20 to 70 W which correspond to most popular incandescent lamps of 60 to 200 W, but the lamp efficacy was disappointingly low. For instance, a lamp of 50 W exhibited a lamp efficacy of as low as about 35 lm/W. On the other hand, the inventors found that if an ellipsoidal arc envelope is used, the lamp efficacy can be increased by 3 to 4% as compared with the case when a straight arc tube is used when the average wall loads (to be defined below) are the same. However, in order to provide high color-rendition, high-pressure sodium lamps which are quite satisfactory in practice, the inventors had to make further extensive studies and experiments.